Many key telephone systems employ loudspeaking intercom arrangements that allow a subscriber to communicate with other subscribers via a special microphone and loudspeaker assembly furnished in addition to a conventional station set. Some of the earlier prior art systems required the subscriber at the called station to initially activate the loudspeaker and microphone arrangement in the telephone set to answer an intercom call. This was found to be unsatisfactory because the called subscriber would often be engaged in an activity or located just out of reach of the telephone set such that activating the intercom arrangement was inconvenient. As a result, later systems having a single intercom line allowed the subscriber at the calling station to activate the loudspeaker and microphone of the called station, thus allowing the called subscriber to respond hands-free. In this type of system, the hands-free answer station contained circuitry that would sense a ringing condition at the called station in order to activate the hands-free answer mode.
A unique problem associated with the hands-free answer operation in a multiple intercom link key telephone arrangement is the difficulty in determining which intercom line a call is originating on. In addressing this problem, one arrangement of interest using multiple intercom links in U.S. Pat. No. 4,063,047 issued on Dec. 13, 1977 to W. A. Huryn. This arrangement uses a common selector circuit to which called station activation digits are transmitted from the calling station. This combines with another circuit that monitors a flashing lamp signal to determine which intercom line is currently in the active calling state such that upon the joint detection of ringing signals and lamp flashing signals, the hands-free answer circuit at the selected call station is activated.
Another arrangement of interest using multiple intercom links is U.S. Pat. No. 3,342,944 issued on Sept. 19, 1967 to R. E. Barbato et al. in which calling and called stations are first interconnected by a single signal link and thereafter transferred to one of a plurality of talking links.
In prior art systems, there is usually a considerable amount of logic circuitry associated with the multiple intercom line arrangement. Such extensive logic is necessary because of the complexity of the circuits involved in connecting to a correct one of the multiple intercom lines for hands-free answer conversation. In addition, in implementing the hands-free answer operation, those prior art arrangements having station sets with assigned intercom numbers must have each calling station transmit digital data signals associated with the station to be called whenever a hands-free answer connection is desired.
It is desirable at times that a subscriber at a station be able to rapidly establish a hands-free answer call to other subscribers within a multiple intercom line system without the dialing of any station numbers. In accordance with the present invention, such service may be provided for any or all stations requiring it with minimally complex circuitry. Summary of the Invention
A key telephone station is equipped with a hands-free answer intercom (HFAI) module which allows for hands-free answer operation. Also included are microphone-off and do-not-disturb features which respectively mute a microphone within the module and block all incoming intercom calls to the station. An intercom call is originated in the standard manner by a station user going off-hook on an idle intercom line and depressing a direct station selection (DSS) button corresponding to a station being called. Depressing the DSS button activates the HFAI module in the called station.
Since the key telephone system has multiple intercom lines, it is not apparent at the station being called which intercom line the station user has selected for the hands-free answer call. It is therefore necessary for the HFAI module at the called station to determine which of the intercom lines the user has selected and connect a talk path thereto. This is accomplished through use of an out-of-the-voice frequency band signaling circuit incorporated in the HFAI module. The HFAI module generates its own out-of-band signal that is applied sequentially to each intercom line in the system. When applied to the intercom line to which the calling station is connected, a closed loop exists and this signal will be returned to the HFAI module. This closed loop consists of the intercom line path connecting the two stations, the side tone path and a preamplifier within the calling station, and the activated DSS signaling lead between the calling station and the HFAI equipped called station.
The closed loop system relaxes the design requirements on both an out-of-band oscillator and detector in the HFAI module since the detector is not required to recognize the signals from other oscillators in the key telephone system. Also, utilization of the out-of-band signal for line selection allows for the interrogating of an intercom line that is in use. The frequency of the out-of-band signal is beyond the frequency response of the handset receiver and loudspeaker used in the key telephone system. Therefore, the HFAI module may interrogate each line without disturbing an on-going conversation thereon by other parties. The interrogation continues until it reaches the intercom line selected by the party at the calling station.
When the HFAI module detects the return of the out-of-band signal on an intercom line, it seizes that line as the appropriate one for HFAI communications. An alerting tone is then placed on the intercom line and is heard over the handset receiver of the calling station and loudspeaker of the called station informing the parties that the HFAI call has been established. The calling party operates in a push-to-talk and release-to-listen mode. When the calling party speaks into the station handset while pushing the DSS button, the called party receives the call over the loudspeaker in the station set. But when the calling party releases the DSS button, the microphone and associated amplifier in the HFAI module are connected to the intercom line allowing the called party to respond completely hands-free. The HFAI call is terminated either by the calling party hanging up or by the called party coming off-hook on the intercom line being used.